Electric power steering apparatus

ABSTRACT

An electric power steering apparatus that enables implementation of a desired steering torque without being affected by a state of a road surface by controlling a steering torque so as to become a value corresponding a steering angle and a steering angular velocity. The apparatus includes a SAT compensation value calculating section that calculates a SAT compensation value based on a SAT value; and a steering reaction compensation value calculating section that calculates a steering reaction compensation value based on the SAT compensation value, a steering angle and a steering angular velocity, and corrects the current command value by the steering reaction compensation value. Further, an electric power steering apparatus controls a twist angle of a torsion bar so as to follow a value corresponding to a steered angle.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. patent application Ser. No. 15/324,813,filed Jan. 9, 2017, in the U.S. Patent and Trademark Office, which is aNational Stage of International Application No. PCT/JP2015/072136 filedAug. 4, 2015, which claims priority from Japanese Patent Application No.2014-169511 filed Aug. 22, 2014, and Japanese Patent Application No.2014-169512 filed Aug. 22, 2014, the contents of all of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an electric power steering apparatusthat drives a motor by a current command value calculated on the basisof a steering torque and a vehicle speed, and provides a steering systemof a vehicle with an assist torque, and in particular to ahigh-performance electric power steering apparatus that enablesimplementation of a steering torque equivalent to a steering angle and asteering angular velocity or to a steered angle without being affectedby a state of a road surface by correcting the current command valueusing a steering reaction compensation value calculated on the basis ofthe steering angle, the steering angular velocity and a self-aligningtorque (SAT) compensation value, or by controlling a twist angle of atorsion bar so as to follow a value corresponding to the steered angle.

BACKGROUND ART

An electric power steering apparatus (EPS) which provides a steeringsystem of a vehicle with an assist torque by means of a rotationaltorque of a motor, applies a driving force of the motor as the assisttorque to a steering shaft or a rack shaft by means of a transmissionmechanism such as gears or a belt through a reduction mechanism. Inorder to accurately generate the assist torque, such a conventionalelectric power steering apparatus performs feedback control of a motorcurrent. The feedback control adjusts a voltage supplied to the motor sothat a difference between a current command value and a detected motorcurrent value becomes small, and the adjustment of the voltage suppliedto the motor is generally performed by an adjustment of duty commandvalues of pulse width modulation (PWM) control.

A general configuration of the conventional electric power steeringapparatus will be described with reference to FIG. 1. As shown in FIG.1, a column shaft (a steering shaft or a handle shaft) 2 connected to asteering wheel 1 is connected to steered wheels 8L and 8R throughreduction gears 3, universal joints 4 a and 4 b , a rack-and-pinionmechanism 5, and tie rods 6 a and 6 b, further via hub units 7 a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10for detecting a steering torque of the steering wheel 1, and a motor 20for assisting a steering force of the steering wheel 1 is connected tothe column shaft 2 through the reduction gears 3. The electric power issupplied to a control unit (ECU) 30 for controlling the electric powersteering apparatus from a battery 13, and an ignition key signal isinputted into the control unit 30 through an ignition key 11. Thecontrol unit 30 calculates a current command value of an assist commandon the basis of a steering torque Th detected by the torque sensor 10and a vehicle speed Vel detected by a vehicle speed sensor 12, andcontrols a current supplied to the motor 20 by means of a voltagecontrol value Vref obtained by performing compensation or the like tothe calculated current command value. A steering angle sensor 14 is notessential, it does not need to be provided, and it is possible to obtainthe steering angle from a rotation sensor such as a resolver connectedto the motor 20.

A controller area network (CAN) 40 exchanging various information of avehicle is connected to the control unit 30, and it is possible toreceive the vehicle speed Vel from the CAN 40. Further, it is alsopossible to connect a non-CAN 41 exchanging a communication,analog/digital signals, a radio wave or the like except with the CAN 40to the control unit 30.

In such an electric power steering apparatus, the control unit 30 mainlycomprises a CPU (including an MPU and an MCU), and general functionsperformed by programs within the CPU are, for example, shown in FIG. 2.

Functions and operations of the control unit 30 will be described withreference to FIG. 2. As shown in FIG. 2, the steering torque Th from thetorque sensor 10 and the vehicle speed Vel from the vehicle speed sensor12 are inputted into a current command value calculating section 31. Thecurrent command value calculating section 31 calculates a currentcommand value Iref1 on the basis of the steering torque Th and thevehicle speed Vel and by using an assist map or the like. The calculatedcurrent command value Iref1 is added in an adding section 32A to acompensation signal CM from a compensating section 34 for improving acharacteristic. A current limiting section 33 limits a maximum value ofthe current command value Iref2 to which the compensation signal CM hasbeen added. The current command value Irefm of which the maximum valuehas been limited is inputted into a subtracting section 32B, where adetected motor current value Im is subtracted from the current commandvalue Irefm.

Proportional integral (PI) control to a deviation I (=Irefm−Im) which isthe subtraction result in the subtracting section 32B is performed in aPI control section 35. The voltage control value Vref obtained by the PIcontrol is inputted into a PWM control section 36, which calculates dutycommand values, and PWM-drives the motor 20 through an inverter circuit37 by means of a PWM signal. The motor current value Im of the motor 20is detected by a motor current detector 38, and is inputted and fed backto the subtracting section 32B. Further, a rotation sensor 21 such as aresolver is connected to the motor 20, and a steering angle θ isoutputted.

The compensating section 34 adds a detected or estimated self-aligningtorque (SAT) to an inertia compensation value 342 in an adding section344, further, adds a convergence control value 341 to the additionresult in an adding section 345, and outputs the addition resultobtained in the adding section 345 as the compensation signal CM to theadding section 32A so as to improve a characteristic of the currentcommand value.

Thus, the conventional electric power steering apparatus detects thesteering torque that a driver manually adds as a twist torque of atorsion bar by means of the torque sensor, and mainly controls the motorcurrent as an assist current corresponding to the torque. Therefore, thesteering torque may vary in accordance with the steering angle by adifference of a state of a road surface (for example, a slope, a low μroad, etc.).

THE LIST OF PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent No. 5074971 B2-   Patent Document 2: Japanese Patent No. 5208894 B2

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

What is shown in the publication of Japanese Patent No. 5074971 B2(Patent Document 1) is a prior art proposing an electric power steeringapparatus that achieves the steering torque equivalent to the steeringangle without being affected by the state of the road surface. Theelectric power steering apparatus disclosed in Patent Document 1suppresses a change of a hysteresis width while maintaining on-centerfeeling of the steering torque by considering a reaction torque on asteering shaft (which is the SAT). In order to make steering feelinggood, the electric power steering apparatus comprises a steering shaftreaction torque detecting means for detecting the reaction torque on thesteering shaft and a steering state judging means for judging at least areturning state, and corrects a basic assist command value on the basisof the reaction torque on the steering shaft so as to increase the basicassist command value when the state is judged as the returning state.

However, the electric power steering apparatus disclosed in PatentDocument 1 has a problem that it does not enable implementation of thesteering torque corresponding to the steering angle and a steeringangular velocity without being affected by the state of the road surfacebecause it deals with the reaction torque on the steering shaft (whichis the SAT) and does not consider the steering angle and the steeringangular velocity.

Further, an electric power steering apparatus shown in the publicationof Japanese Patent No. 5208894 B2 (Patent Document 2) has a problem thatsmoothness is bad, its operation is not stable and followability to atarget value is not good since the electric power steering apparatusmakes a calculation result by proportional integral (PI) control to adeviation between a target value of a steering torque (a twist angle)and the steering torque to be a current command value.

The present invention has been developed in view of the above-describedcircumstances, and the object of the present invention is to provide anelectric power steering apparatus that enables implementation of asteering torque equivalent to a steering angle and a steering angularvelocity without being affected by a state of a road surface bycontrolling the steering torque so as to become a value corresponding tothe steering angle and the steering angular velocity or by controlling atwist angle of a torsion bar so as to follow a value corresponding to asteered angle.

Means for Solving the Problems

The present invention relates to an electric power steering apparatusthat assists and controls a steering system by driving a motor based ona current command value calculated based on a steering torque and avehicle speed, the above-described object of the present invention isachieved by that comprising: a SAT compensation value calculatingsection that calculates a SAT compensation value based on a SAT value;and a steering reaction compensation value calculating section thatcalculates a steering reaction compensation value based on the SATcompensation value, a steering angle and a steering angular velocity;wherein the electric power steering apparatus corrects the currentcommand value by the steering reaction compensation value.

The above-described object of the present invention is more effectivelyachieved by that wherein the steering reaction compensation valuecalculating section comprises a virtual spring constant gain sectionthat inputs the steering angle and calculates a spring componentcompensation value, a virtual damper constant gain section that inputsthe steering angular velocity and calculates a damper componentcompensation value, an adding section that calculates a spring-dampercomponent compensation value by adding the spring component compensationvalue and the damper component compensation value, and a subtractingsection that calculates the steering reaction compensation value bysubtracting the spring-damper component compensation value from the SATcompensation value; or wherein the SAT compensation value calculatingsection comprises a SAT estimating section that obtains the SAT valuebased on the steering torque, a motor angular velocity, a motor angularacceleration and the current command value, a filter that eliminatesnoise of the SAT value, and a gain section that outputs the SATcompensation value by multiplying an output of the filter by a gain; orwherein the filter and the gain section are vehicle speed sensitive; orwherein the virtual spring constant gain section and the virtual damperconstant gain section are vehicle speed sensitive; or wherein thecorrection is adding the steering reaction compensation value to thecurrent command value.

Further, the present invention relates to an electric power steeringapparatus that drives a motor based on a motor current command value,and assists and controls a steering system by driving and controllingthe motor, the above-described object of the present invention isachieved by that wherein the electric power steering apparatuscalculates the motor current command value by calculation for twistangle control based on a deviation between a target twist anglecorresponding to a steered angle and an actual twist angle of thesteering system and the actual twist angle, and controls the actualtwist angle so as to follow a value corresponding to the steered angle.

The above-described object of the present invention is more effectivelyachieved by that wherein the target twist angle is calculated by a twistangle table inputting the steered angle; or wherein the twist angletable is vehicle speed sensitive; or wherein the calculation for twistangle control is performed by a subtracting section that obtains adeviation between the target twist angle and the actual twist angle, aposition control section that outputs the target twist angular velocityby performing positon control of the deviation, a differentiatingsection that differentiates the actual twist angle, and a velocitycontrol section that inputs the target twist angular velocity and anoutput of the differentiating section and performs velocity control; orwherein the velocity control section performs at least one among Pcontrol calculation, I control calculation and D control calculation, orcombinations of them; or wherein a limiter is provided in a post-stageof the velocity control section; or wherein the electric power steeringapparatus adds a current command value of assist control, a currentcommand value of a SAT estimation value, or a current command value forrestraint of steering wheel vibration to the motor current commandvalue.

Effects of the Invention

The electric power steering apparatus of the present invention enablesconstant steering feeling to be obtained without being affected by astate of a road surface since it considers the SAT and performscorrection of a reaction corresponding to the steered angle (thesteering angle) and the steering angular velocity. The electric powersteering apparatus enables implementation of a desired steering torquebecause of controlling the steering torque so as to become a valuecorresponding to the steering angle and the steering angular velocity.

Further, the electric power steering apparatus of the present inventionenables implementation of the steering torque equivalent to the steeredangle and enables constant steering feeling to be obtained without beingaffected by the state of road surface since it controls a twist angle ofa torsion bar so as to follow a value corresponding to the steeredangle. Position control to the deviation between the target value of thetwist angle and the actual twist angle and velocity control to thetarget twist angular velocity and the twist angular velocity using atleast one of PID controls enable smooth and stable operation, and thereare advantages that followability to the target value is good and asteady deviation barely remains.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a configuration diagram illustrating a general outline of anelectric power steering apparatus;

FIG. 2 is a block diagram showing a configuration example of a controlsystem of the electric power steering apparatus;

FIG. 3 is a block diagram showing a configuration example (a firstembodiment) of the present invention;

FIG. 4 is a block diagram showing a configuration example of a SATcompensation value calculating section;

FIG. 5 is a block diagram showing a configuration example of a steeringreaction compensation value calculating section;

FIG. 6 is a characteristic diagram showing an example of characteristicsof a virtual spring constant gain section;

FIG. 7 is a characteristic diagram showing an example of characteristicsof a virtual damper constant gain section;

FIG. 8 is a flowchart showing an operating example (the firstembodiment) of the present invention;

FIGS. 9A, 9B and 9C are characteristic diagrams showing the operatingexample (the first embodiment) of the present invention compared with aprior art;

FIG. 10 is a block diagram showing a configuration example (a secondembodiment) of the present invention;

FIG. 11 is a block diagram showing a configuration example of a twistangle control section;

FIG. 12 is a flowchart showing an operating example (the secondembodiment) of the present invention;

FIG. 13 is a characteristic diagram of a steered angle showing theoperating example (the second embodiment) of the present invention;

FIG. 14 is a characteristic diagram of a twist angle showing theoperating example (the second embodiment) of the present invention;

FIG. 15 is a characteristic diagram of a steering torque showing theoperating example (the second embodiment) of the present invention; and

FIG. 16 is a characteristic diagram showing another example of a twistangle table.

MODE FOR CARRYING OUT THE INVENTION

In order to obtain constant steering feeling by achieving a steeringtorque equivalent to a steering angle (a steered angle) and a steeringangular velocity or to the steered angle without being affected by astate of a road surface, the present invention achieves a desiredsteering torque respectively by obtaining a spring-damper componentcompensation value by means of the steering angle and the steeringangular velocity, in addition, calculating a steering reactioncompensation value on the basis of a SAT compensation value and thespring-damper component compensation value, and correcting a currentcommand value by means of the steering reaction compensation value, orby controlling a twist angle of a torsion bar so as to follow a valuecorresponding to the steered angle.

In the present invention, it is possible to obtain the constant steeringfeeling without being affected by the state of the road surface becauseof performing correction of a reaction corresponding to the steeredangle (the steering angle) and the steering angular velocity,considering a SAT being a road-surface reaction. Further, the presentinvention enables stable control with high accuracy because ofcalculating a motor current command value through a position controlsection and a velocity control section including at least one of PIDcontrols using a deviation between a target value of a twist anglecalculated depending on the steered angle and an actual twist angledetected from the torsion bar.

Hereinafter, an embodiment of the present invention will be describedwith reference to the accompanying drawings.

FIG. 3 shows a configuration example (a first embodiment) of the presentinvention. A control unit 100 comprises a torque system control section110, shown by a broken line, that performs control by using a steeringtorque Th, a vehicle speed Vel, a steering angle θ, a steering angularvelocity (a motor angular velocity) w and a steering angularacceleration (a motor angular acceleration) a, and a motor systemcontrol section 120, shown by a dashed line, that performs controlrelated to driving a motor 20 by means of a motor driving section 23comprising an inverter or the like.

The torque system control section 110 comprises a current command valuecalculating section 111, a differential control section 112, a yaw rateconvergence control section 113, a robust stabilization compensatingsection 114, a self-aligning torque (SAT) compensation value calculatingsection 150 and a steering reaction compensation value calculatingsection 160, and includes adding sections 116A and 116B and asubtracting section 116C. Further, the motor system control section 120comprises a compensating section 121, a disturbance estimating section122, a motor angular velocity calculating section 123, a motor angularacceleration calculating section 124 and a motor characteristiccompensating section 125, and includes adding sections 126A and 126B.

The steering torque Th is inputted into the current command valuecalculating section 111, the differential control section 112, the yawrate convergence control section 113 and the SAT compensation valuecalculating section 150. The vehicle speed Vel is inputted into thecurrent command value calculating section 111, and the yaw rateconvergence control section 113 inputs the vehicle speed Vel as aparameter. The current command value calculating section 111 calculatesa current command value Ia on the basis of the steering torque Th andthe vehicle speed Vel. The differential control section 112 has afunction that enhances responsiveness of control near a neutral positionof a steering and achieves smooth steering. An output of thedifferential control section 112 is added to the current command valueIa in the adding section 116A, and a current command value Ib that isthe addition result is inputted into the SAT compensation valuecalculating section 150, and is inputted into the adding section 116B.

The yaw rate convergence control section 113 inputs the steering torqueTh and the steering angular velocity ω, and puts a brake on a motionthat a steering wheel sways and turns in order to improve convergence ofa yaw of a vehicle. A yaw rate signal YR of the yaw rate convergencecontrol section 113 is added to the current command value Ib in theadding section 116B, and a current command value Ic that is the additionresult is inputted into the robust stabilization compensating section114.

Further, the SAT compensation value calculating section 150 inputs thesteering torque Th, the current command value Ib from the adding section116A, the steering angular velocity w from the motor angular velocitycalculating section 123 and the steering angular acceleration α from themotor angular acceleration calculating section 124, estimates a SATvalue, performs signal processing of the estimated SAT value by using afilter and a gain section, and outputs a SAT compensation value ISATthat gives appropriate information of a road surface to the steeringwheel as reaction force. The SAT compensation value ISAT is inputtedinto the steering reaction compensation value calculating section 160,the steering angle θ and the steering angular velocity w have been, inaddition, inputted into the steering reaction compensation valuecalculating section 160, and a calculated steering reaction compensationvalue ISKC is inputted into the adding section 116C.

Further, the current command value Ic obtained by addition of the addingsection 116B is inputted into the robust stabilization compensatingsection 114. The robust stabilization compensating section 114 is, forexample, a compensation section shown in Japanese Published UnexaminedPatent Application No. H8-290778 A, eliminates a peak value in aresonance frequency of a resonance system consisting of an inertiaelement and a spring element, included by a detected torque, andcompensates a phase shift of the resonance frequency that impairsresponsiveness and stability of a control system. A current commandvalue Id being an output of the robust stabilization compensatingsection 114 is added to the steering reaction compensation value ISKC inthe adding section 116C, and an added current command value Ie isinputted into the adding section 126A within the motor system controlsection 120.

The motor angular velocity calculating section 123 within the motorsystem control section 120 calculates the steering angular velocity (themotor angular velocity) w on the basis of an inter-motor-terminalvoltage Vm and a motor current Im, and the steering angular velocity wis inputted into the motor angular acceleration calculating section 124,the yaw rate convergence control section 113 and the SAT compensationvalue calculating section 150. The motor angular accelerationcalculating section 124 calculates the steering angular acceleration αon the basis of the inputted steering angular velocity w, and thesteering angular acceleration α is inputted into the motorcharacteristic compensating section 125 and the SAT compensation valuecalculating section 150. A motor characteristic signal Ima from themotor characteristic compensating section 125 is added to the currentcommand value Ie in the adding section 126A, and a current command valueIf being the addition result is inputted into the compensating section121 consisting of a differential compensator or the like. A signalobtained by adding an output of the disturbance estimating section 122in the adding section 126B to a current command value Ig compensated inthe compensating section 121 is inputted into the motor driving section23 and the disturbance estimating section 122.

The disturbance estimating section 122 is such an apparatus as shown inJapanese Published Unexamined Patent Application No. H8-310417 A,enables maintenance of a motor control characteristic desired in outputcriteria of the control system, and does not allow losing stability ofthe control system, based on a current command value 111 obtained byadding the output of the disturbance estimating section 122 to thecurrent command value Ig that is a control target of a motor output andis compensated in the compensating section 121, and based on the motorcurrent Im.

FIG. 4 shows a configuration example of the SAT compensation valuecalculating section 150, which adds the current command value Ibcorresponding to an assist torque Tm and the steering torque Th in anadding section 151, subtracts a signal obtained by multiplying thesteering angular acceleration α by a motor inertia J from the additionresult in a subtracting section 152, and, moreover, subtracts a signalobtained by multiplying a static friction Fr by a positive or negativesign of the steering angular velocity w from the subtraction result in asubtracting section 153. The subtraction result of the subtractingsection 153 is a SAT estimation value *SAT (for example, shown inJapanese Published Unexamined Patent Application No. 2008-18825 A). TheSAT estimation value *SAT is inputted into a filter 154 that issensitive to a vehicle speed and has a frequency characteristic,moreover, is multiplied by a gain in a gain section 155 being sensitiveto a vehicle speed, so that the SAT compensation value ISAT is obtained.

Moreover, the filter 154 is a phase delay filter having a gain thatmakes the magnitude of the SAT estimation value *SAT decreased to anecessary and sufficient value as a static characteristic gain. Further,the gain section 155 has a function that makes the SAT compensationvalue ISAT small in such a case that importance of information about aroad surface is relatively low as static steering or a low speedrunning.

FIG. 5 shows a configuration example of the steering reactioncompensation value calculating section 160. The steering angle θ isinputted into a virtual spring constant gain section 161, for example,having a vehicle speed sensitive characteristic shown in FIG. 6, and thesteering angular velocity w is inputted into a virtual damper constantgain section 162, for example, having a vehicle speed sensitivecharacteristic shown in FIG. 7. A spring component compensation valueIKm from the virtual spring constant gain section 161 and a dampercomponent compensation value ICm from the virtual damper constant gainsection 162 are added in an adding section 163, and a spring-dampercomponent compensation value IKCm being the addition result issubtraction-inputted into a subtracting section 164. The SATcompensation value ISAT from the SAT compensation value calculatingsection 150 has been addition-inputted into the subtracting section 164,the subtraction result (=ISAT-IKCm) of the subtracting section 164 isoutputted as the steering reaction compensation value ISKC, and thesteering reaction compensation value ISKC is inputted into the addingsection 116C.

Moreover, the virtual spring constant gain section 161 is sensitive tothe vehicle speed in FIG. 6, but may not be sensitive to the vehiclespeed. Similarly, the virtual damper constant gain section 162 issensitive to the vehicle speed in FIG. 7, but may not be sensitive tothe vehicle speed.

An operation example (the first embodiment) of such a configuration willbe described with reference to a flowchart shown in FIG. 8.

First, the steering torque Th, the vehicle speed Vel, the steering angleθ and the steering angular velocity w are inputted (Step 51). Thecurrent command value Ib is calculated in the current command valuecalculating section 111, the differential control section 112, etc.(Step S2), in addition, the yaw rate signal YR is calculated in the yawrate convergence control section 113, and the current command value Icto which the yaw rate signal YR has been added is outputted as thecurrent command value Id through the robust stabilization compensatingsection 114.

The current command value Ib and the steering angular acceleration αcalculated in the motor angular acceleration calculating section 124 areinputted into the SAT compensation value calculating section 150 (StepS3), and the SAT estimation value *SAT is calculated in the addingsections 151 and 152 and the subtracting section 153 (Step S4). The SATestimation value *SAT is subjected to the filter processing and the gainprocessing in the filter 154 and the gain section 155 (Step S5), and theSAT compensation value ISAT is outputted and inputted into the steeringreaction compensation value calculating section 160 (Step S6).

The steering reaction compensation value calculating section 160calculates the spring component compensation value IKm in the virtualspring constant gain section 161 depending on the steering angle θ (StepS10), calculates the damper component compensation value ICm in thevirtual damper constant gain section 162 depending on the steeringangular velocity w (Step S11), moreover, adds the spring componentcompensation value IKm and the damper component compensation value ICmin the adding section 163, and calculates and outputs the steeringreaction compensation value ISKC by subtracting the spring-dampercomponent compensation value IKCm being the addition result from the SATcompensation value ISAT (Step S12).

The steering reaction compensation value ISKC is inputted into theadding section 116C, is added to the current command value Id, moreover,is added to the motor characteristic signal Ima in the adding section126A, and drives the motor 20 through the compensating section 121 andthe motor driving section 23 (Step S20). The above operations arerepeated until termination (Step S21).

FIGS. 9A, 9B and 9C are characteristic diagrams showing the operatingexample (the first embodiment) of the present invention. In the case ofchanging the steering angle θ in accordance with a steering patternshown in FIG. 9A, a prior art causes a characteristic having a largedifference shown by the solid line (a normal road) and the broken line(a slippery road surface in rainy weather etc. or a low μ road) in FIG.9B, however, the present invention causes an almost constantcharacteristic regardless of a road surface as shown by the solid line(the normal road) and the broken line (the slippery road surface inrainy weather etc. or the low μ road) in FIG. 9C because of controllingthe steering torque so as to become a value corresponding the steeringangle and the steering angular velocity by the compensation of thesteering reaction.

Moreover, the SAT compensation value may be calculated by directlyprocessing a value obtained by detecting force on a rack shaft or torqueon an intermediate shaft in the filter and the gain section. Further,the steering angle may be detected by an angle sensor installed on thesteering wheel side or an angle sensor installed on the column side, ormay be calculated by means of a motor angle.

Next, a second embodiment will be described that achieves a desiredsteering torque by controlling a twist angle of a torsion bar so as tofollow a value corresponding to a steered angle in order to obtainconstant steering feeling by achieving a steering torque equivalent tothe steered angle.

FIG. 10 shows the second embodiment of the present invention. A steeredangle θh is inputted into a twist angle table 210, and a target twistangle Δθref calculated in the twist angle table 210 depending on thesteered angle θh is inputted into a twist angle control section 220. Anactual twist angle Δθ of a torsion bar in an EPS (steering) system 200is inputted into the twist angle control section 220. The twist anglecontrol section 220 calculates a motor current command value Irefdepending on a deviation between the target twist angle Δθref and theactual twist angle Δθ, and drives the EPS (steering) system 200 on thebasis of the calculated motor current command value Iref so that theactual twist angle Δθ becomes the target twist angle Δθref.

The twist angle table 210 outputs the target twist angle Δθref dependingon the inputted steered angle θh. That is, the twist angle table 210outputs the target twist angle Δθref that gradually increases in thepositive or negative direction as the steered angle θh increases in thepositive or negative direction. It is desirable that the outputcharacteristic is nonlinear as shown in FIG. 10, but it is possible thatthe output characteristic is linear.

The twist angle control section 220, into which the target twist angleΔθref from the twist angle table 210 and the actual twist angle Δθ ofthe torsion bar are inputted, is, for example, configured as shown inFIG. 11.

That is, the target twist angle Δθref is addition-inputted into asubtracting section 221, the actual twist angle Δθ issubtraction-inputted into the subtracting section 221, at the same time,is inputted into a differentiating section 223 and istime-differentiated. The deviation between the target twist angle Δθrefand the actual twist angle Δθ that is calculated in the subtractingsection 221 is inputted into a position control section 222 having aposition control gain Kpp, and is multiplied by the position controlgain Kpp, so that a target twist angular velocity Δωr is obtained. Thetarget twist angular velocity Δωr is addition-inputted into asubtracting section 224-1 within a velocity control section 224 of anext stage. The differential value of the differentiating section 223 issubtraction-inputted into the subtracting section 224-1 within thevelocity control section 224, and, at the same time, is inputted into aproportional control calculating section 224-3 having a proportionalcontrol gain Kvp.

The subtraction result of the subtracting section 224-1 is inputted andintegrated in an integral control calculating section 224-2 having anintegral control gain Kvi, and the integral control result SC1 isaddition-inputted into a subtracting section 224-4. Further, theproportional control result SC2 of the proportional control calculatingsection 224-3 is subtraction-inputted into the subtracting section224-4. A maximum value of a current value Jr being the subtractionresult (=SC1−SC2) in the subtracting section 224-4 is limited in alimiter 225, and the motor current command value Tref whose maximumvalue is limited is outputted. The motor current command value Trefbecomes a command value of the EPS system 200, and the motor is driven.

An operation example (the second embodiment) of such a configurationwill be described with reference to a flowchart shown in FIG. 12.

First, the steered angle θh is inputted into the twist angle table 210(Step S30). The twist angle table 210 calculates the target twist angleΔθref corresponding to the steered angle θh, and makes the target twistangle Δθref inputted into the subtracting section 221 (Step S31).Further, the actual twist angle Δθ is detected in the torsion bar withinthe EPS system 200 (Step S32), and is inputted into the subtractingsection 221. The deviation between the target twist angle Δθref and theactual twist angle Δθ that is calculated in the subtracting section 221(Step S33), the deviation is inputted into the position control section222, and the target twist angular velocity Δωr is calculated bymultiplying the deviation by the position control gain Kpp (Step S34).

Further, the actual twist angle Δθ from the EPS system 200 istime-differentiated in the differentiating section 223 (Step S35), thisdifferential result and the target twist angular velocity Δωr from theposition control section 222 are inputted into the velocity controlsection 224, and calculation of velocity control is performed (StepS36). The velocity control section 224 of the present embodiment has anI-P control structure, the maximum value of the current value Jrobtained by the I-P control is limited in the limiter 225 (Step S37),and the EPS system 200 is driven by means of the obtained motor currentcommand value Tref (Step S38). The above operations are repeated untiltermination.

The velocity control section 224, first, calculates the deviationbetween the target twist angular velocity Δωr and the differentialresult from the differentiating section 223 in the subtracting section224-1, multiplies the deviation by the integral control gain Kvi andintegrates it, and addition-inputs the integral control result SC1 intothe subtracting section 224-4. Further, the velocity control section 224inputs the differential result into the proportional control calculatingsection 224-3, performs the proportional control calculation bymultiplying the differential result by the proportional control gainKvp, subtraction-inputs the proportional control result SC2 into thesubtracting section 224-4, and obtains the current value Jr bycalculating “the integral control result SC1 minus the proportionalcontrol result SC2” in the subtracting section 224-4.

FIGS. 13, 14 and 15 are characteristic diagrams showing the operatingexample of the second embodiment. In the case of changing the steeredangle θh in accordance with the characteristic shown in FIG. 13, thetarget twist angle Δθref shown by a thin line in FIG. 14 is outputtedfrom the twist angle table 210. The actual twist angle Δθ (shown by athick line) operates so as to follow the target twist angle Δθref (shownby the thin line), and the result of this is that a steering torqueoccurring as hand feeling becomes as shown in FIG. 15.

Moreover, the above-mentioned twist angle table 210 has thecharacteristic corresponding to only the steered angle θh, but it ispossible for the characteristic to be sensitive to the vehicle speed soas to change depending on the vehicle speed V. As shown in FIG. 16, inthe case of the vehicle speed sensitive table, the characteristic isthat the target twist angle Δθref, as a whole, increases as the vehiclespeed V increases. Further, it is possible to provide a phasecompensating section in the pre-stage or the post-stage of the twistangle table 210, and it is also possible to add a current command valueof conventional assist control to the motor current command value of thetwist angle control. Moreover, it is also possible to add a currentcommand value of a SAT estimation value or a current command value forrestraint of steering wheel vibration to the motor current command valueof the twist angle control. The velocity control section can use notonly the above-mentioned I-P control structure, but also PI control, Pcontrol, PID control, or PI-D control.

EXPLANATION OF REFERENCE NUMERALS

-   1 steering wheel-   2 column shaft (steering shaft, handle shaft)-   10 torque sensor-   12 vehicle speed sensor-   13 battery-   20 motor-   23 motor driving section-   30, 100 control unit (ECU)-   31 current command value calculating section-   35 PI control section-   36 PWM control section-   110 torque system control section-   111 current command value calculating section-   120 motor system control section-   122 disturbance estimating section-   150 SAT compensation value calculating section-   160 steering reaction compensation value calculating section-   161 virtual spring constant gain section-   162 virtual damper constant gain section-   200 EPS (steering) system-   210 twist angle table-   220 twist angle control section-   222 position control section-   224 velocity control section

1. An electric power steering apparatus that assists and controls asteering system by driving a motor based on a current command valuecalculated based on a steering torque and a vehicle speed, comprising: aSAT compensation value calculating section that calculates a SATcompensation value based on a SAT value; and a steering reactioncompensation value calculating section that calculates a steeringreaction compensation value based on said SAT compensation value, asteering angle and a steering angular velocity, wherein said steeringreaction compensation value calculating section comprises a virtualspring constant gain section that inputs said steering angle andcalculates a spring component compensation value, a virtual damperconstant gain section that inputs said steering angular velocity andcalculates a damper component compensation value, an adding section thatcalculates a spring-damper component compensation value by adding saidspring component compensation value and said damper componentcompensation value, and a subtracting section that calculates saidsteering reaction compensation value by subtracting said spring-dampercomponent compensation value from said SAT compensation value andwherein said electric power steering apparatus corrects said currentcommand value by said steering reaction compensation value from saidsteering reaction compensation value calculating section.
 2. Theelectric power steering apparatus according to claim 1, wherein said SATcompensation value calculating section comprises a SAT estimatingsection that obtains said SAT value based on said steering torque, amotor angular velocity, a motor angular acceleration and said currentcommand value, a filter that eliminates noise of said SAT value, and again section that outputs said SAT compensation value by multiplying anoutput of said filter by a gain.
 3. The electric power steeringapparatus according to claim 2, wherein said filter and said gainsection are vehicle speed sensitive.
 4. The electric power steeringapparatus according to claim 1, wherein said virtual spring constantgain section and said virtual damper constant gain section are vehiclespeed sensitive.
 5. The electric power steering apparatus according toclaim 2, wherein said virtual spring constant gain section and saidvirtual damper constant gain section are vehicle speed sensitive.
 6. Theelectric power steering apparatus according to claim 3, wherein saidvirtual spring constant gain section and said virtual damper constantgain section are vehicle speed sensitive.
 7. The electric power steeringapparatus according to claim 1, wherein said correction is adding saidsteering reaction compensation value to said current command value.